Add video tag helper
Add video tag helper

https://github.com/lotus/lotus/issues/396

This commit exposes a `video` method to the view
which can be used to generate HTML5 video tags.
It accepts options to generate HTML attributes along
with a block for creating fallback content and/or
`source` and `track` tags.

lotus/assets,hanami/assets,hanami/assets,hanami/assets,hanami/assets,vyper/assets